Disallow methods not in a contract scope to call methods in contract scope

[kevaundray]

Problem

Methods and state variables in a contract scope have functionality and semantics that does not apply when you are not speaking about contracts.

Consider the following program copied from Jake's awesome PR:

fn main(x : Field, y : pub Field) {
     constrain Foo::xy(x) == Foo::triple(y);
 }

 contract Foo {
      y : Field
     fn double(x: Field) -> Field { x * 2 }
     fn xy(x: Field) -> Field { x * y }
 }

Calling xy outside a contract brings forth a few question:

• What does it mean when one calls xy and accesses the state variable y in this context?
• Does this behaviour differ in anyways if xy was called by another contract?
• Can a regular Noir program modify the state of y?

I think we could possibly define semantics for this however since there is no usecase for it and it would lead to additional complexity, its better to not allow it now and allow it in the future if we need to.

Proposed solution

Noir contracts can call other contracts and programs. However, Noir programs cannot call Noir contracts.

Alternatives considered

No response

Additional context

No response